When an existing telephone switching exchange in a central office is to be replaced by a new exchange, the cable pair or line of each subscriber must be disconnected from the old exchange and connected to the new exchange in such manner as to avoid service interruptions or inconvenience to the subscribers at or after cutover.
To this end, the subscriber data is collected into a computer data base and this information is compared to the data base proposed for the new exchange. The proposed data base is then corrected, if necessary, to conform to the subscriber data. Testing then ensues to determine if the subscriber equipment then in use actually conforms to the subscriber data.
One test that must be performed is the ringing frequency test. Some subscriber telephones will only respond to a twenty-five Hertz signal; other telephones will only respond to a fifty Hertz signal and still other telephones will respond only to other preselected frequencies. If a ringing signal of incorrect frequency is sent to a subscriber, the telephone will not ring and the subscriber will not know that a call is to be received. Accordingly, it is necessary to perform call verification of the ringing frequencies of subscribers telephones by placing calls using the old exchange. Test equipment using a ringing interface card would be employed to detect seven cycles of the ringing frequency in order to measure the ringing frequency. (The time period which corresponds to seven cycles is so short that the subscribers telephone will not respond and the subscriber will not know that a test is being made). This arrangement was designed for testing private line service using metallic ringing.
However, certain exchanges now being replaced provided multi-party service using a divided ringing system, and such service must continue to be provided when a new exchange is installed. The subscribers lines which are connected between the exchange and the telephone are cable pairs having a tip conductor and a ring conductor. In a divided ringing system, some ringing signals are applied between the tip conductor and ground while others are applied between the ring conductor and ground. Hence, for multi-party service, it is necessary not only to measure the ringing frequency but also to detect which of the tip and ring conductors receives the ringing signal.
The present invention is directed toward a new type of ringing interface card which overcomes the inability of known cards for testing multi-party service and which enables a technician to measure the ringing frequency and simultaneously to detect which of the tip and ring conductors receives the ringing signal.